The reading of projection X-ray images, also referred to as radiographs, is a tedious and exhausting procedure as they typically come in batches and time pressure is high. Radiologists are trained to visually follow a certain path in their reading in order not to oversee details that may turn out diagnostically relevant. For example, for a posterior-anterior chest radiograph, most attention is directed to analyze the lung fields for so as to determine soft tissue findings, and to the cardio-vascular system. But also bone-related findings may be of importance, examples of which include metastases, cysts, or small fractures.
In the example of the posterior-anterior chest radiograph, in order to rule out bone lesions during the image reading, each rib has to be followed along its extent from the spine via the lateral turn point to the anterior tip of the rib. In the posterior-anterior chest radiograph, this path describes a curve. Along this curve, a number of other structures are crossing that are overlaid in the projection image. These are mainly other neighboring ribs, but also clavicles, the lung field border, the shadow of the breast, the heart shadow, and others. These overlaid structures visually distract the reader from following the rib and identifying any lesions associated with the rib. It is noted that similar problems occur also in other types of radiographs which involve other types of bone objects than ribs.
It is known to unfold the rib cage from a 3D computed tomography (CT) volume into a 2D image, for example from US 2013/070996 A1. It is said that the ability to unfold the CT volume data from 3D to 2D may help doctors to locate rib lesions more easily and accurately, since small lesions are typically difficult to identify and locate in the 3D CT volume, but easier to locate in a corresponding 2D unfolded image. Similar techniques appear also to be known from US 2013/101197 A1, citing WO 2006/050102.
Disadvantageously, the above identified ‘rib unfolding’ method is only applicable to 3D CT volume data and cannot be applied to projection X-ray images.